Microphone damping system having rear openings



Feb. 6, 1951 s. c. TALLMAN MICROPHONE DAMPING SYSTEM HAVING REAR OPENINGS Filed March 18, 1949 l nin",

/NVE/vron S. C. 714./ LMAN ATTORNEY Patented Feb. 6, 1951 BHCROPHONE DAlVIPING SYSTEM HAVING REAR OPENINGrSr` Samuel C. -Tal1man,.Chatham, Ni J i, assigner to Bell Telephone Laboratories, Incorporated, New York,V N. Y., va. corporation of New York.

Application` March 18, 1949, Serial No. 82,100

6` Claims.'

This invention relates to diaphragm damping systems for acoustic .devices4 and more lparticularly to acoustic resistance assemblies 4for transmitters and receivers Vespecially suitable for. use in hand telephones.

It is well known to provide acoustic dampingl of a diaphragm by a controlledacoustic leakto an air chamber adjacent the diaphragm. This air leakage path to the a-ir chamberv may be of several types. Morrison et al. Patent 2,220,942, issued November 12, 1942, disclosesvand describes one such acousticv element-in which one or more strips of acoustic silk fabric arelaid over an aperture in the plate behind the diaphragm. Where the acoustic damping is provided by a plurality of such apertures over which acoustic resistance material is laid, the manufacture of the transmitter or receiver is complicated as each aperture must be carefully assembled to make certain that the proper aperture area is provided for so that the resistance-dampingof-each device is the same as that of the others.

One general object of this invention is to obtain an improved acoustic resistance unit that maybe easily and rapidly assembled and has a prescribed acoustic resistance.

A further object of this-invention is to'expedite and facilitate themanufacture of transmitters and receivers.

A still further object of this invention is to prevent undesirable acoustic leakage between the parts of the acoustic resistanceunit.

In accordance with oneV feature of this invention, a sheet or strip of acoustic resistance material is placed between two4 apertured. members Whose apertures are so proportioned thatA the total area of air passages through the cloth is of a preassigned constant magnitude-irrespective of the angular orientation of the two members.

The invention andthe above-notedv and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a plan View ofan acoustic resistancel unit illustrative of one embodimentl of this in- Vention;

Fig. 2 is a side view, partly in section, of a transmitter unit illustrative ofonevembodiment of this invention;

Fig. 3 is an enlarged siderview, in section, of a portion of the transmitter unitV of Fig. 2, showing the acoustic resistance unit; and

Fig. 4 is a partial sectional andexploded perspective` viewshowing the method' of assembly of the acoustic resistance unit of the transmitter shown in Fig. 2.

Referring now to the drawing, the acoustic resistance assembly illustrated in Fig. l comprises an annular member I I having a plurality of apertures I2 therein, an annularly shaped sheet of acoustic resistance` material Illawhich may be a` silk or nylon fabric, placed ontop of fmembervI'I,

and al second annular member I4 having a plu-- rality of apertures I5 therein placed on top ofthe acoustic resistance material I3 The' superm posed annular members `have the same center and their respective apertures I2 and I5 have-,the same mean radii and radial Width. The. aper tures I2 and I5 are formed so that their sidesare portions of the cir-cumferences of circles` drawn with the center of the annular members as their. center. Thus when the'annular members-IVI and I4 are properly superimposed so that theircenters are coincident, the apertures are also Vsuperimposed.

In accordance with a feature of this invention, the apertures are so constructed and arranged that the total area of air passages. through the acoustic resistance materialis constantfregardfless of the angular orientation of each-of the :annular members. apertures isV the same their areas can be. ex-

any circumference.

I5 by the anglev that nhl-H3) :360 degrees` (l)` where n is the number of apertures |51 Representing the angual length of the largerapertures I2 by the angle 0, thev apertures I2 are dimensioned'so that n9=360 degrees. '(3) Accordingly, if' N represents the number of apertures I2 in the annular member IIthen-=the total aperture length in member I I Y can be repre-4 sentedgby N0. However N thus represents the total circumferential length of apertureof member lIfthat'isblockedf bythe member I4; while'Na represents-thetotal circumferential length of air passage through the two annular members; or through the'acoustic resistance material` |13. It is readily apparentthat Since the radial Width of. thel this total length Na is not dependent on the relative angular orientation of the annular members I I and I4. In assembling the unit, therefore, the top annular member I4 may be dropped rapidly on the acoustic resistance material I3 and the lower annular member I I in any angular relation to member II without altering the acoustic resistance damping of the unit.

This may also be seen by noting that if the upper apertured member I4 is rotated, half of the apertures I5 in the member I4 which at present coincide with apertures I2 in the member II will begin to close; simultaneously, the other half of the apertures I5 will open by the same amount. As one set closes off another set opens. Therefore, since the area of the openings is independent of the angular position of the member I4 with respect to member I I, the acoustic resistance will be independent of angular position also.

Typical values I have used are, for example;

a=18 degrees =12 degrees :30 degrees 11,:12 N=4 Theoretically, the ends of the apertures I2 and I should be radial lines; I have found however, that the actual variation in resistance caused by making the ends of the apertures circular is very small and that production of the parts is facilitated. The outside diameters of the annular members are held in position by other equipment to automatically superimpose the centers of the two annular members; thus the outside diameters may conveniently be made the same and the members centered at the time the unit is clamped, cemented, or in some other way joined together.

Referring to Fig. 2, there is shown a transmitter unit illustrative of one embodiment of this invention. The transmitter, which may be ofthe general type disclosed in the application Serial No. 783,324, fil-ed October 31, 1947, now Patent No. 2,532,694, granted December 5, 1950, of H. W. Bryant, comprises a circular bridge or foundation member I8 having a fiat annular seating surface I9, a lightweight metallic diaphragm 20 the peripheral portion of which is seated upon the surface I9, a Ihemispherical electrode 2I affixed centrally to the diaphragm 2l) and vibratile therewith, a back or fixed electrode 22, carbon granules 23 substantially filling the chamber bounded by the two electrodes, and a metallic cap 24 crimped over the back electrode. The diaphragm 20, together with a moisture-resistant screen or membrane 21 and a multiapertured cap or cover 28, is secured to the bridge or foundation member I3 by the bottom portion of the ferrule or annular band 29.

The bridge member I3 has a plurality of apertures 3l therein communicating with the chamber behind the diaphragm 23; these apertures 3I correspond to the apertures I2 in Fig. l. A ring of damping material or fabric 32 is held in place on the bridge member I8 by an annular member 33, which has a plurality of apertures 34 therein, corresponding to the apertures I5 of Fig. 1. The annular member 33 is held nrmly in place by the top portion 35 lof the ferrule 29, which also serves to hold the transmitter assembly together.

I have found that the acoustic leakage between the bridge and the fabric ring is reduced by making the radial length of this leakage long and by making the back surface of the bridge member I8 convex. Thus if the/bridge has a slight convex surface, such as for example three degrees convex, the force produced by clamping the at annular member 33 on the bridge is sufficient to prevent excessive leakage between the fabric ring 32 and the bridge.

In the assembling of this acoustic resistance unit during the manufacture of the transmitter. the transmitter unit is assembled rst. As shown in Fig. 4 the moisture-resistant screen 2l', diaphragm 20 and bridge I8 are all first properly assembled with the'ferrule 29. The acoustic resistance ring 32 and annular member 33 are then dropped into place on top of the bridge member I8 without concern for their relative angular positions. -The upper portion 35 of the ferrule 29 is then bent over. The outside diameter of the annular member 33 is chosen so that the member 33 is centered with respect to the bridge i8 by the crimping tool during the bending back of the ferrule to make the mean circumference of the apertures 34 in the annular member coincide exactly with that of the apertures 3l in the bridge.v Constant acoustic resistance is thus assured for each assembly without the delay and ineiiciency of carefully assembling each acoustic resistance unit individually.

It is understood Ythat, the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. In an .acoustic device comprising a diaphragm, means adjacent said diaphragm for controlling the vibration thereof, said means coinprising a first member having therein a number of apertures arrangedrin a circle, acoustic resistance material extending across said apertures, and a second member overlying said rst member and having therein a number of apertures arranged in a circle substantially coincident with said first circle, the angular length of one of said first apertures being equal to the angular length of one of said second apertures and the distance between saiddsecond apertures.

2. An acoustic device comprising a diaphragm and means defining an acoustic network coupled to said diaphragm for controlling the vibration thereof, said means comprising a first flat circular member opposite one face of said diaphragm and having therein a number of equally dimensioned aperturesarranged in a circle, 'acoustic resistance material overlying said apertures, and a second flat circular member overlying said rst member, having the same center therewith, and having therein a dierent number of equally dimensioned apertures arranged in a circle, the radius of said second circle being substantially equal to the radius of said rst circle and said circles being substantially centered at the center of said circular members, said first and second apertures being of the saine radial width and having sides approximating portions of the circumferences of circles centered at the center of said members, the angular length of one of said first apertures being equal to the angular length of one of said second apertures and the distance between said second apertures, the total area of acoustic material exposed through the apertures being the same irrespective of the relative angular orientation of said first and second members.

3. In a telephone transmitter, a foundation member, a diaphragm adjacent said foundation member and defining a chamber therewith, said foundation member having therein a plurality of apertures arranged substantially in a circle, acoustic resistance material extending across said apertures, a second member overlying said foundation member and having therein a different number of apertures arranged in a circle substantially coincident with said rst circle, the angular length of one of said apertures of said foundation member being equal to the angular length of one of said apertures of said second member and the angular length between said second apertures, and means joining said foundation member, said acoustic resistance material, and said second member together.

4. A telephone transmitter comprising a foundation member, a diaphragm adjacent said foundation member and defining a chamber therewith, said foundation member having therein a number of apertures arranged in a circle, acoustic resistance material extending across said apertures, a second member overlying said foundation member and having therein a different nurnber of apertures arranged in a circle substantially coincident with said rst circle, one angular dimension of the apertures of one member being equal to one angular dimension of the apertures of the other member and the angular distance between the apertures of the other member so that the total area of said acoustic resistance material exposed through the apertures is the same irrespective of the relative angular orientation of said foundation and second members, and means joining said foundation member, said acoustic resistance material, and said second member together 5. A telephone transmitter in accordance with claim 4 wherein the surface of the foundation member adjacent the acoustic resistance material 4 is convex.

6. A telephone transmitter comprising an annular foundation member, a diaphragm adjacent said member and defining a chamber therewith, said foundation member having therein a number of equally dimensioned apertures arranged in a circle, a ring of acoustic resistance material adjacent said foundation member and overlying said apertures, a iiat annular member overlying said foundation member and having therein a different number of equally dimensioned apertures arranged in a circle, said rst and said second circles being substantially coincident, said first and second apertures being of the same radial width and having sides approximating portions of circumferences of circles centered at the center of said above-mentioned circles, the angular length of one of said rst apertures being equal to the angular length of one of said second apertures and the distance between said second apertures, the total area of acoustic material exposed through said apertures being the same irrespective of the relative angular orientation of said foundation and flat annular members, and means comprising a ferrule joining said foundation member, said ring, and said ilat member together.

SAMUEL C. TALLMAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,653,727 Oberst Dec. 27, 1927 1,757,938 Hayes May 6, 1930 2,196,342 Ruttenberg Apr. 9, 1940 FOREIGN PATENTS Number Country Date 388,348 Great Britain Feb. 23, 1933 494,293 Great Britain Oct. 19, 1938 

